Scaling down of conventional civilian turbofan engines, however, also presents difficulties due mainly to the disproportionate scaling of certain factors, such as strength to weight and tolerances. For example, turbofan engines typically have a segmented case assembly, mainly for weight reduction reasons, but also to facilitate fabrication and assembly. A conventional case assembly 200 is illustrated in FIG. 1, and includes a fan case 202, an intermediate case 204, a compressor case 206, a gas generator case 208, a turbine case 210 and a turbine exhaust case 211 about centreline 212. The gas generator case 208, turbine case 210 and turbine exhaust case 211 surround the hot section of the engine and are typically made of steel or nickel alloys, which have good thermal resistance properties. However steel is relatively heavy, and therefore cooler portions such as the intermediate case 204 and the compressor case 206 typically employ lighter materials such as magnesium and/or aluminium. Steel is conventionally used for the fan case 202 because its strength is desirable for containing blade-off events.
A similar prior art configuration 300 is illustrated in FIG. 2, a case assembly 300 (only the upper half of which is shown), having a fan case 344, an intermediate case 346, and a gas generator case 352 (the turbine and exhaust cases are not shown) bolted together, along centreline 312. A compressor shroud 348 for encircling the compressor blades is bolted to the intermediate case 346, as is a bearing seat (not shown) at location 357. Flange connections 302, 304 and 306 are provided to accommodate differences in thermal expansion rates amongst the different material case components. Typically the case components are assembled in stages, as the engine component top-level assemblies are assembled therein. A splitter 342 extends forwardly of struts 340 in there intermediate case 346. Intermediate case 346 is typically cast, with struts 340 and splitter 342 integrally cast therein. However, casting is a process which is sometime difficult to control, and which requires minimum wall thickness to achieve good quality—which poses problems when considering the design of very small turbofan engines, since small cast structures are disproportionately heavy. Improvement in case design is desired.